A phenol-resin based sliding material had been used before filing of Patent Document 1, Japanese Patent No. 2517604. The present applicant proposed in Patent Document 1 a sliding material, which replaces the former sliding material and consists of 20 to 90% by weight of at least one of polyimide and polyamide-imide, 5 to 60% by weight of graphite, and 0.5 to 20% by weight of a friction-adjusting agent consisting of clay. Patent Document 1 describes the graphite as follows.
Graphite bonded with polyimide or polyamide-imide mainly imparts improved friction characteristics to the sliding material. Graphite used for such a purpose may be either synthetic or natural. Particle shape of graphite may be granular or flat. From a view point of wear resistance, graphite has preferably 250 μm or less of particle diameter. A crystalline property of the graphite is expressed in terms of d(002) plane distance measured by X ray. From the view point of wear resistance, a preferable distance is 3.50 angstroms or less. The graphite having the plane distance mentioned above is liable to cleave at intervals of the distances mentioned above. When graphite is flaky or has scale form, flat major surfaces of the graphite are aligned on the surface of sliding material. Therefore, the area of the graphite that is on the surface of sliding material is large, and coefficient of friction is advantageously reduced. Graphite is used in an amount of 5 to 60%. W hen this amount is less than 5%, coefficient of friction of the sliding material is so high and hence wear amount is large. On the other hand, when this amount exceeds 60%, bonding strength of resin and bonding strength between the backing metal and sliding layer are weakened, so that the amount of wear increases. The amount of use is preferably 30 to 60%.
Patent Document 2, Japanese Patent No. 3026269 relates to a polyamide-imide resin based sliding material proposed by the present applicant. Heat-treated resin particles, which are essentially individually separated from each other, are dispersed in an amount of 5 to 80% by weight in the aromatic polyamide-imide of the sliding material. Carbon may be added as an optional component. The carbon is described as follows.
Carbon improves wear resistance and decreases coefficient of friction. The carbon can be any one of such amorphous carbons as carbon black, coke powder, and glass-like carbon, and crystalline carbons such as synthetic carbon or natural graphite (graphite). Amorphous carbon is recommended in the light of wear resistance, while crystalline carbon is recommended in light of friction characteristics. Therefore, either amorphous carbon or crystalline carbon is used depending upon the application. When the carbon content is less than 1%, neither wear-resistance nor friction characteristics is effectively improved. On the other hand, when the carbon content exceeds 60%, the mechanical properties are impaired, and coefficient of friction is liable to be instable due to carbon separation. Therefore, the carbon content must be 1 to 60%. Carbon content is preferably 5 to 50%. Average particle diameter of carbon is preferably 250 μm or less. W hen coarse carbon having average particle diameter more than 250 μm and fine carbon are compared with each other, provided that the carbon content is identical for both cases, the exposed area of the former carbon on the sliding surface is less than that of the latter carbon, which is not effective for improving sliding properties. Average particle diameter of carbon is preferably 10 to 40 μm.
Patent Document 3, Japanese Unexamined Patent Publication (kokai) No. Hei 5-331314 proposes a heat-resistant resin sliding material consisting of 40 to 95% by weight of heat-resistant resin such as polyimide resin, and 5 to 60% by weight of spherical graphite having 3 to 40 μm of average particle diameter. Resin-based spherical particles are fired in an inert gas atmosphere or under vacuum to graphitize the same. The spherical graphite and heat resistant resin are blended to provide a composition of the sliding material. The spherical graphite is described as follows.
The spherical graphite herein has a uniform particle diameter, and it is 3 to 40 μm in average. Highly geometrically spherical graphite is preferred. Starting material of the spherical graphite is preferably at least one of phenol resin, naphthalene resin, furan resin, xylene resin, divinylbenzene polymer, and styrene-divylbenzene copolymer. A production method of such spherical graphite comprises emulsion polymerizing these starting materials by known method to form spherical particles, and firing these spherical particles under an inert gas atmosphere, such as nitrogen gas or argon gas, or under vacuum. As a result, carbonization and/or graphitization occur and the spherical graphite is obtained.
The technical level of the sliding material based on graphite-added resin is revealed from Patent Documents 1 through 3 and is illustrated from the following points of view.    (a) Graphite is a material having laminar crystalline structure, in which (002) planes are superimposed. The interlayer slipping is liable to occur. These properties are utilized in such a manner that cleavage planes of graphite are oriented to the sliding direction (Patent Document 1).    (b) As the degree of graphitization increases, the graphite becomes closer to natural graphite, which is soft and well lubricating. W hen the degree of graphitization is low, a resultant hard carbon is added as hard particles to enhance the wear resistance and to adjust friction. Since approximately geometrically spherical graphite proposed in Patent Document 3 is as hard as Hv800 to 1200, it is believed to be hard carbon.    (c) Approximately geometrically spherical graphite is obtained by firing resin, as described in, for example, Patent Documents 2 and 3. Natural graphite and synthetic graphite have conventionally been used for sliding materials. Contrary to the case of fired resin, since the shapes of natural graphite and synthetic graphite are considerably deformed from spherical graphite, these graphites have small particle ratio.